Bert's WWWboard - Coaxial tweeter with MD2http://forum.bd-design.nl/
Forum at BD-DesignenCoaxial tweeter with MD2 (reply)I remember reading abour someone making hexagonal enclosures with a Pioneer B20 with a tweeter added. You should be able to google it.

There are some press-in tweeters available now that are incredibly small, that person put them on top of a wooden phase plug. I think they Tweeter was made by Dayton.

There are three problems with this concept in high end audio:
I doubt they will have the efficiency to match your MD2.
Connecting wires to such a tweeter will be either difficult or ugly.
Time alignment will not be very good.

I was thinking about give a try to a pair of tweeter with my oris 250 and MD2 speakers.

Did someone have an idea of a tweeter that fit in the center of the aer ?

The only tweeters that fit are those used for car audio, there is not much space in there!. Problem with those tweeters will be the very low sensitivity, very low quality and besides that, you will create some sort of phase plug which will probably phase out more then you think.

Bottom line....that idea won't work (thought of that by myself years ago for Lowthers).